1. Field of the Invention
The present invention relates to a charging apparatus provided with a magnetic brush that contacts a body to be charged, with this charging apparatus being suitably applied, in particular, to an image forming apparatus that develops an electrostatic latent image formed on an image bearing member, such as a photosensitive member or a dielectric member, using developer and records the developed latent image on a sheet or the like.
2. Related Background Art
FIG. 8 is a front view showing a construction of a conventional image forming apparatus.
The image forming apparatus shown in FIG. 8 is constructed by a photosensitive drum 1, LED exposing means 2, a corona charger 3, a developing device 4, a cleaner 5, a fixing device 6, a transferring apparatus 7, pre-exposing means 8, a scanner unit 9, an original table 10, a transferring material supplying mechanism 11, a transferring material cassette 12, a main body 13, a discharging roller 14, and a sheet tray 15. The transferring apparatus 7 is constructed by a transferring belt 71, a drive roller 72, a driven roller 73, and a transferring apparatus 74. Also, the unit 9 is constructed by an original illuminating lamp, a short-focus lens array, and a CCD sensor.
When a copy start button is depressed and a copy start signal is inputted into a control portion (not shown), the photosensitive drum 1 starts to rotate and its surface is charged by the corona charger 3 to have a predetermined potential during the rotation. On the other hand, the scanner unit 9 starts to move and emits illuminating and scanning light toward an original G placed on the original table 10. During this operation, reflection light generated by reflection of the illuminating and scanning light by a surface of the original is imaged by the short-focus lens array and is made incident on the COD sensor (including a light receiving portion, a transferring portion, and an outputting portion) A light signal is converted into a charge signal in the light receiving portion of the COD sensor. Then, in the transferring portion, the charge signals are successively transferred to the outputting portion in synchronization with a clock pulse. In this outputting portion, the charge signal is converted into a voltage signal, which is then subjected to amplification and reduction in impedance and is outputted. The obtained analog signal is converted into a digital signal through known image processing and is sent to a printer portion constructed by components of the photosensitive drum 1 to the pre-exposing means 8 incorporated in the main body 13.
In this printer portion, the LED exposing means 2 is turned ON/OFF for light emission in accordance with an image signal received from an original table side, thereby forming an electrostatic latent image corresponding to an original image on the surface of the photosensitive drum 1. Then, the electrostatic latent image on the photosensitive drum 1 is developed by the developing device 4 in which toner particles are contained, thereby forming a toner image on the photosensitive drum 1.
The toner image formed on the photosensitive drum 1 in this manner moves onto the transferring apparatus 7 in accordance with the rotation of the photosensitive drum 1 and is electrostatically transferred onto a transferring material by the transferring apparatus 74. Following this, the transferring material (not shown) is electrostatically separated, is conveyed by the transferring belt 71, and is fed into the fixing device 6. In this fixing device 6, the transferring material that has been fed thereinto is subjected to heat-fixing, thereby heat-fixing and outputting an image.
On the other hand, the surface of the photosensitive drum 1, from which the toner image has been transferred, is subjected to processing where adhering contaminant, such as transfer residual toner, is removed by the cleaner 5. Further, as occasion arises, there is performed exposure by the pre-exposing means 8 for removing a light memory resulting from the image exposure. Following this, the photosensitive drum 1 is applied to the next image formation.
As a material for the photosensitive drum 1, in many cases, there is used an organic photosensitive member, an amorphous silicon-based photosensitive member (hereinafter referred to as the xe2x80x9ca-Si-based photosensitive memberxe2x80x9d), or the like. The a-Si-based photosensitive member has various features. For instance, the a-Si-based photosensitive member is high in surface hardness, exhibits a high sensitivity to a semiconductor laser or the like, and shows almost no degradation even after repetitive usage. Therefore, the a-Si-based photosensitive member is used as a photosensitive member of a high-speed copier, a laser beam printer, or the like.
As a method of charging the a-Si-based photosensitive member, it is possible to use a corona charging system using corona discharging, a roller charging system with which charging is performed through direct discharging using a conductive roller, an injection charging system with which charging is performed by maintaining a sufficient contact area using magnetic particles or the like and directly injecting charges into the surface of a photosensitive member, or the like.
Among these systems, the corona charging system and the roller charging system utilize discharging, so that discharge products tend to adhere to a surface. Also, the a-Si-based photosensitive member has an extremely high surface hardness and is resistant to abrasion, so that discharge products tend to remain on its surface. In addition, a smeared image phenomenon easily occurs when charges move in a surface direction on the photosensitive member surface, on which an electrostatic latent image has been formed, due to the absorption of moisture or the like under high-humidity circumstances or the like. In contrast to this, the injection charging system does not positively use discharging and is a charging system with which charges are directly injected from a portion contacting the photosensitive member surface. Therefore, there hardly occurs the smeared image phenomenon.
In the conventional image forming apparatus, however, the a-Si-based photosensitive member is obtained using a production method with which a gas is plasmanized and solidified using a high-frequency wave or a micro wave and a film is formed by depositing the plasmanized and solidified gas on an aluminum cylinder. Consequently, in the case where the plasma is not caused in a uniform manner, unevenness of film thickness or unevenness of composition occurs in a circumferential direction or a longitudinal direction. Therefore, conventionally, potential unevenness of around several tens of V has occurred in the developing portion. This is caused by a phenomenon where a difference in electrostatic, capacity occurs due to the unevenness of film thickness and therefore there occurs a difference in charging performance as well as a phenomenon where potential decay under a dark state (hereinafter referred to as the xe2x80x9cdark decayxe2x80x9d) between charging and developing due to pre-exposure performed to erase a light memory resulting from previous rotation varies due to a difference in the film thickness or composition and therefore, the potential unevenness is further increased in the developing portion.
In the case where the a-Si-based photosensitive member is used, the dark decay described above becomes extremely large in comparison with a case of an organic photosensitive member even under a dark state, and the potential decay due to the light memory resulting from image exposure is further increased, so that it is required to provide the pre-exposing means 8 before charging in order to erase the light memory resulting from previous rotation. Therefore, the dark decay between charging and developing becomes extremely large and there occurs a potential decay of around 100 to 200 V. At this time, due to the film thickness unevenness or composition unevenness described above, there occurs a potential unevenness of around several tens of V.
If this potential unevenness occurs, the a-Si-based photosensitive member having a large electrostatic capacity is significantly influenced by the unevenness because of its small contrast in comparison with the organic photosensitive member, which leads to a situation where density unevenness becomes prominent.
In view of such a problem, it is effective to use a method with which charging is performed for a plurality of times, for instance. If charging is performed for a plurality of times in order to solve the problem in that the dark decay is increased due to the light memory, it becomes possible to substantially reduce the light memory through charging performed for the first time, which makes it possible to reduce the dark decay after charging is performed for the second time. As a result, a potential ghost or potential unevenness is substantially improved.
Here, if the injection charging system is used when charging is performed for a plurality of times, the potential ghost and the potential unevenness are substantially improved because of its high charging ability and high potential converging property. Also, discharging is hardly used in the case of the injection charging system as described above, so that there hardly occurs the smeared image phenomenon. As an injection charger, it is effective to use a magnetic brush charger that uses magnetic particles, for instance. The magnetic brush charger performs charging using contact points of the magnetic particles, so that there are obtained advantages in that the surface area for charging becomes wide and resistance to pollution is obtained. As a result, it becomes possible to maintain high charging performance even for a long-term use.
In the case where the magnetic brush charger is used, however, there is a problem in that the magnetic particles adhere to the photosensitive member. This is a phenomenon where upon charging, in the case where a large potential difference occurs between a photosensitive member surface and a magnetic particle holding member to which a voltage is applied, the magnetic particles adhere to the photosensitive member and mix into a developing apparatus from the photosensitive member. In particular, this phenomenon easily occurs in end portions of a magnetic particle coating. This is because a portion that can be in contact with the magnetic particles and be charged and a portion that is not capable of contacting the magnetic particles and is not charged coexist in the end portions, so that a large potential difference is partially caused and the adhesion of the magnetic particles to the photosensitive member becomes prominent. On the other hand, there is also used a method with which insulation processing is performed in the end portions of the magnetic particle coating and the potential gradient of the coating end portions is reduced, thereby suppressing the magnetic particle adhesion phenomenon. In this case, however, the magnetic particle coating portions are subjected to the insulation processing, so that there may occur various problems. For instance, a negative effect develops due to a difference in coating amount resulting from a difference in surface property between the magnetic particle holding member and the portions subjected to the insulation processing. Also, the durability of the portions subjected to the insulation processing is decreased due to rubbing with the magnetic particles. Further, although it is possible to suppress the magnetic particle adhesion phenomenon, it is difficult to completely avoid this phenomenon.
An object of the present invention is to provide a charging apparatus and an image forming apparatus with which a body to be charged is charged using a plurality of chargers.
Another object of the present invention is to provide a charging apparatus and an image forming apparatus that prevent a situation where magnetic particles constituting a magnetic brush of a charger adhere to a body to be charged.
Another object of the present invention is to provide a charging apparatus and an image forming apparatus with which even it magnetic particles adhere to a body to be charged, it is possible to recover the magnetic particles by means of an action of magnetic field generating means of a charger.
Another object of the present invention is to provide a charging apparatus and an image forming apparatus with which in the case where a body to be charged is charged using a plurality of magnetic brush chargers, there is solved a problem caused by adhesion of magnetic particles of the magnetic brushes to the body to be charged.
Another object of the present invention is to provide a charging apparatus and an image forming apparatus with which each area of a body to be charged corresponding to one of end portions of a magnetic brush of a magnetic brush charger is charged in advance.
Another object of the present invention is to provide a charging apparatus and an image forming apparatus with which it is possible to recover magnetic particles adhering to each area of a body to be charged corresponding to one of end portions of a magnetic brush of a magnetic brush charger by means of an action of magnetic field generating means.
Another object of the present invention is to provide an image forming apparatus that is suited for a case where amorphous silicon is used as a body to be charged.
Other objects and features of the present invention will become more apparent from the following detailed description to be made with reference to the accompanying drawings.